ScDocument dtor calls ClearLookupCaches(), which calls GetNonThreadedContext().
But ScDocument instances used for copy&paste GetFormatTable() fails
on null mxPoolHelper, because ScDocument ctor doesn't set it in such a case.
So set up the pointer in ScInterpreterContext on demand only if actually
needed.

Change-Id: If3811da5bb00a2d7d404c089ee1bf46037a2cddb
Reviewed-on: https://gerrit.libreoffice.org/68350
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>